Catheters known in the prior art have been used in humans and animals for diagnostic, monitoring and treatment purposes. Such catheters must be small and flexible in order to function without irritating a body part into which they are inserted. Catheters have been used to infuse medications or remove samples of tissue and fluid for analysis. Multi-lumen catheters have been used to infuse medication and remove samples at the same time.
If a sample must be removed for purposes of analysis, and taken to a laboratory for subsequent analysis, the delay in performing analysis and transmitting the data back to a doctor sometimes can be fatal to a patient. Catheters have also been used to form hydraulic columns for transmitting pressure readings to an external sensor. In pressure sensing catheters, the hydraulic column may have problems with air bubbles, kinks in the tubing, and blood clots affecting the reliability and accuracy of critical readings.
Catheters used for monitoring variations in blood pressure within a blood vessel include those using catheter tip transducers insertable into a blood vessel with the transducer providing direct pressure monitoring by transducing blood pressure at the region of interest. Such catheters have used a semiconductor material constructed and arranged with strain sensing beams for creating a proportionate electrical signal representative of the monitored pressure and transmitting the signal by electrical conductors through the length of the catheter to meters or the like located externally of the monitored body.
An example of such a catheter is seen in U.S. Pat. No. 4,274,423. This catheter includes a pressure sensor disposed within the end portion of a catheter. The pressure sensor uses a pressure sensitive diaphragm constructed from block of semiconductor material. The diaphragm is located adjacent a side port in the catheter housing connected to the end of the catheter with the port providing access to the pressure environment. The diaphragm is directly deflected relative to the pressure exerted thereon and the deflection is sensed by one or more strain gauges located within the diaphragm itself. The strain gauges are connected by conductors to a processor circuit located external of the catheter.
A concern with such catheters is to provide a transducer which is small, while also being sufficiently responsive to pressure variations to provide meaningful electrical output signals. The semiconductor block with the diaphragm or membrane of the type employed in U.S. Pat. No. 4,274,423, may well have a width on the order of 1.2 mm, which limits the size of the catheter and, hence, its application for use in measuring blood pressure and other conditions within a blood vessel.
U.S. Pat. No. 4,722,348, shows a pressure sensing catheter of small size, however, since the catheter body shown has side port pressure inlet apertures which are depressed into the catheter body, this catheter is susceptible to air bubbles being trapped within the pressure apertures, resulting in inaccurate sensor measurements. This catheter is also connected to a detector circuit located outside of the catheter.